This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present disclosure. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present disclosure. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
1. Field of the Invention
The present inventions relate to the field of wellbore operations. More specifically, the inventions relate to the removal of filter cake from a wellbore, and methods for stimulating a hydrocarbon-producing well using a thermal technique. The present inventions further relate to the use of nano-particles to enable rapid localized heating of a filter cake, and alternatively to assist in the controlled curing of cement.
2. General Discussion of Technology
In the drilling of oil and gas wells, a wellbore is formed using a drill bit that is urged downwardly at a lower end of a drill string. During the drilling process, a drilling fluid is placed in the bore of the drill string. The drilling fluid is typically referred to as drilling mud, or just “mud.”
The drilling mud is injected into the bore of the drill string to increase hydrostatic pressure at the bottom of the wellbore. This, in turn, controls the flow of formation fluids into the wellbore. The drilling mud also helps to keep the drill bit cool and clean during drilling. In this respect, the drilling mud is circulated down the drill string and through orifices in the drill bit. The drilling mud is then circulated back up the annular region between the drill string and the surrounding borehole. Circulation of the drilling mud in this manner allows drill cuttings to be moved away from the interface between the drill bit and the rock face, and brings the drill cuttings to the surface for analysis and/or disposal.
There are three main categories of drilling fluids: water-based muds, non-aqueous muds, and gaseous drilling fluid. Non-aqueous muds, sometimes referred to as non-aqueous fluids (NAFs), are muds where the base fluid is an oil. Environmental considerations aside, NAFs are often preferred over water-based muds and gaseous drilling fluids, as NAFs generally offer increased lubrication of the drill bit and drill string. NAFs also help stabilize shale formations more effectively than do water-based or gaseous muds. NAFs also withstand greater heat without breaking down and beneficially tend to form a thinner filter cake than water-based muds.
There are two general categories of NAFs: oil-based muds (OBMs) and synthetic-based muds (SBMs). A common example of a base fluid for an OBM is diesel oil. SBMs use a synthetic oil rather than a natural hydrocarbon as the base fluid. An example of a base fluid for a SBM is palm oil. SBMs are most often used on offshore rigs as SBMs have the beneficial properties of an OBM, but lower toxicity. This is of benefit when the drilling crew is working in an enclosed area, as may be the case on an offshore drilling rig operating in an arctic environment. Also, in some parts of the world, cuttings from wells drilled with certain types of SBMs can be discharged to the sea, and thereby may reduce the cost of cuttings disposal.
The drilling fluid used for a particular job is generally selected to avoid formation damage. For example, in various types of shale formations, the use of conventional water-based muds can result in a deterioration and collapse of the formation. Similarly, muds made from fresh water can cause clays in a sandstone or other type formation to swell and dislodge. This, in turn, can negatively affect the permeability of the sandstone near the wellbore. The use of an oil-based formulation circumvents these problems.
NAFs are also preferred in deviated or horizontally completed wellbores. The non-aqueous-based fluid provides a slick film along which tubular bodies and equipment may glide while moving across non-vertical portions of the wellbore.
As noted, a conventional oil-based drilling mud formulation is comprised basically of oil. Examples of oil include diesel oil and mineral oil. An OBM will also include a wetting agent. An example of a common wetting agent is dodecylbenzene sulfonate. An OBM may also include a thickener, or “viscosification agent.” Examples of viscosification agents are amine-treated clays such as bentonite. Neutralized sulfonated ionomers have also been proposed as viscosification agents.
A NAF will also include a water phase. This typically represents sodium chloride or calcium chloride brine. The NAF will also then include a surfactant as an emulsifying agent. An example of a surfactant is an alkaline soap of fatty acids. The surfactant aids in blending the base oil with the brine and stabilizing the continuous oil emulsion. Finally, a weighting agent may be used. An example of a weighting agent is barite or barium sulfate.
As a wellbore is drilled through a permeable, hydrocarbon-bearing formation, the drilling mud will form a “filter cake.” The filter cake from a NAF is comprised primarily of water droplets, weighting agent particles, and drilled cuttings previously suspended in the drilling mud. The filter cake forms a thin, low-permeability layer along permeable portions of the borehole. Beneficially, the filter cake at least partially seals permeable formations exposed by the bit. This helps prevent the loss of the liquid portion (or filtrate) of the drilling fluids into the formations during the wellbore forming process. The filter cake also helps prevent the surrounding rock matrix from sloughing into the wellbore. Of note, the drilling process can be ongoing for days or even weeks.
A low-permeability filter cake is also desirable for running completion equipment in the wellbore. For example, it is sometimes desirable to run the completion hardware in a clear brine to prevent solids plugging of a sand control screen. The mud filter cake prevents the completion brine from rapidly leaking off to the formation as the completion hardware is run. In addition, a low-permeability filter cake helps prevent the gravel used in a gravel pack from bridging off during gravel placement due to a loss of hydration in the slurry.
Near the end of the completion process for a well, it is desirable to remove the filter cake from the wellbore. In this respect, the filter cake not only prevents the invasion of drilling fluids and completion fluids into a formation, but reciprocally can hinder the movement of hydrocarbon fluids into the completed well. Further, because fine particles within the drilling mud may also penetrate into pores residing in the near wellbore region, the particles can provide an even greater barrier to flow. Thus, the productivity of a well is somewhat dependent on the operator being able to effectively remove the filter cake and wash the formation. To do this, the operator may circulate an oxidizer, an enzyme, or an acid solution through the formation to remove fine clay particles remaining from the drilling mud.
In some instances, merely circulating an acid (or other) solution into the wellbore does not completely remove the filter cake. In this instance, the hydrocarbons must at least partially flow through or around the filter cake or cause some of the filter cake to “lift off” of the wellbore face. At the same time, if some or all of the filter cake lifts off of the wellbore face, this represents a plugging hazard. For example, in the case of an open-hole completion, the filter cake pieces may plug sections of a sand control screen.
It is noted that the per-foot surface area of the filter cake is greater than that of the sand screen. Therefore, if only a small percentage of the filter cake is transferred to a sand screen, significant plugging can occur, requiring production fluids to travel a tortuous path into the production tubing.
A need therefore exists for a wellbore filter cake that can be disintegrated within the wellbore at an appropriate time after drilling. In addition, a need exists for a filter cake that can be energized downhole to increase a temperature within the filter cake to a temperature that is greater than an initial (e.g., ambient, local, or well-bore) circulation temperature of the filter cake and to cause substantial degradation of the filter cake. Further, a need exists for a drilling fluid composition that may be used to form a filter cake and which contains the means for degradation upon exposure to electromagnetic wave energy and/or thermal energy. Still further, a need exists for well completion methods that involve the use of drilling fluids, stimulation fluids, or cement that have nano-particles placed therein.